This invention relates to a system and method by which a set of portable devices can cooperatively determine their position or improve an estimate of their position.
It is becoming increasingly common for portable devices to be provided with one or more techniques by which they may determine their geographical location. For some devices, such as hand-held GPS receivers, their primary function is to accurately determine their position and provide this information to the user. For other devices, such as mobile telephones, it can be useful for the device to be able to at least roughly determine their location. This allows the mobile telephone to make use of location-based services. For example, knowledge of a user's location can allow a smartphone to provide a map of the user's location, allow digital photographs captured with the device to be geotagged, allow webpages to be served that include content relevant to the user's location, and generally allow applications running on the smartphone to make use of the location information.
There are many techniques by which portable devices can determine their location. The use of Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS) or GLONASS system, allow geographical position to be determined to a high degree of accuracy but it is not typically possible for a portable device to receive a signal whilst inside a building or in dense urban areas. Certain improvements to the GPS system, such as Assisted GPS (A-GPS), are used that increase the likelihood of a device determining its position in difficult environments. A-GPS allows a mobile network enabled device to download almanac and/or ephemeris information for the GPS satellites from a central server and can provide an accurate time to the device. However, A-GPS does not improve the likelihood that a GPS-enabled device can determine its location if that device cannot receive signals from at least four different satellites. This is a common occurrence in dense urban environments.
Less accurate positioning methods include cellular and wifi location determination methods that can provide a rough location for a portable device through the knowledge that if the device is connected to a particular cell base station or wifi access point then it must be within a particular distance of the known location of that cell base station or Wi-Fi access point. Devices able to detect multiple types of cell base stations, or those of different providers, can improve the resolution of their location estimate through trilateration of their position relative to those base stations and/or access points that are in range. It is advantageous if devices can utilise as many different wireless technologies as possible so as to increase the number of base stations or access points that might be detectable at any point in time. Wireless technologies that can be useful in aiding a mobile device in determining its position include GSM, 3G, LTE, WiMAX, Wi-Fi.
Various refinements of these wireless network-based methods are available that further improve the accuracy of a device's location estimate. In order to determine the distance of the device from the base station or access point, these refinements generally rely on a combination of the received signal strength, the time taken by signals to propagate between the base station and device, and (for cellular systems that utilise a directional antenna) the direction from which the signals are received. Examples of positioning methods that incorporate these refinements include E-Cell ID and E-OTD.
Despite their lower accuracy, cellular and Wi-Fi location determination methods are often more reliable indoors and in dense urban environments than satellite-based location methods. Portable devices that are to perform well in both outdoor environments that offer good reception of navigation satellite signals and in indoor or dense urban environments must generally support a multitude of communication technologies. For example, smartphones typically include a GPS receiver, one or more mobile network transceivers (e.g. GSM, 3G), and often a Wi-Fi transceiver (e.g. IEEE 802.11n).
However, not all devices include multiple radio transceivers and many are not equipped to determine their location in indoor or urban environments—for example, GPS receivers designed for use in the wilderness. Other devices often do not include any means by which they can determine their location, yet would benefit from knowing their location—for example, digital cameras could be configured to geotag photos with the location at which they were taken were the camera to receive such information.
There is therefore a need for improved methods by which portable devices can determine their location, particularly in indoor and dense urban environments.